1. Field of the Invention
This invention relates to depositing different materials on a substrate, for example, for the purpose of making quarter wave plate optical reflectors of high quality, such as are required in the manufacture of ring laser gyroscopes.
2. Description of the Related Art
One known method for doing this is described in a paper entitled "Advances in Low-Energy Ion Beam Technology" by W. Laznovsky, published in the journal "Vacuum Technology", August 1975. The method described in that paper uses a Kaufman-type ion source which directs a beam of low energy Argon ions onto a "target" made of the substance to be deposited on a substrate. The effect is to dislodge, or sputter, particles of the target material which are then deposited on the substrate to form a thin layer. When it is desired to produce alternate layers of different materials, such as would be required when manufacturing quarter-wave plate optical reflectors or filters, it is necessary to change the targets, and the aforementioned paper envisages a system employing three such targets that can be interchanged.
A disadvantage of previously proposed systems such as that described above is that, during the interval between the deposition of each layer, the substrate can become coated by contaminants. These may be removed using ions of low energy from a secondary ion gun but that adds to the complexity of the apparatus and of its operation.
Another problem is that the environment required for deposition of one material may not be ideal for the deposition of another material. In GB Patent Specification 2,020,701B it is proposed that the pressure of the low vacuum gas inside the chamber, the gas being deliberately introduced to control the stoichiometry of the deposited material, should be altered between the processes of depositing the different materials. The need to make such adjustments adds, however, to the complexity and time required between depositing the different layers and where a large number of layers are to be deposited on a large number of substrates, the process may become very time-consuming and expensive. Experiments, using systems such as have been described, have shown them to be unreliable in that the quality of the resulting products has been inconsistent to an extent that could require a significant proportion of them to be discarded if required for use in ring laser gyroscopes as previously mentioned.